A canine hook

[Rock of the Fraternity]

If you haven't seen any of the "I am your puppy"-videos on YouTube, this hook probably won't make sense.

If you have seen it, consider this cruel twist in the Demiplane of Dread: the adorable, fluffy puppy that someone has taken into their home actually starts to dominate their life. Not just by the usual (barking, whining, looking cute, making a mess), although it does that as well. At first.
If disciplined, or if it is simply ignored, the puppy turns to different ways of establishing control.

First begins the gaslighting; this creature someone has taken into their home turns out to really be capable of speech. It talks to its 'owner' in the dead of night, saying horrible things, reciting the unfortunate's sins. When other people are present, the creature only barks and looks innocent, but whenever the 'owner' is alone, the canine steps up its verbal assault. As only the 'owner' hears the dog talk, it's easy for them to assume they're going insane.

If gaslighting does not break the victim's will, the dog proceeds to use supernatural powers, inflicting suggestions on the 'owner' so they do whatever the puppy wants of them. It avoids this, since actual magic can be detected, and that puts the dog itself at risk.

Surprisingly, it is passing rare for 'owners' to actually be killed by the creature that abused their kindness and the rules that surround being a guest in someone else's home. Many of the victims would wish it were not so; the puppies don't refrain from killing because they hold any affection for their temporary hosts, they refrain because breaking a victim's sanity and running away is much safer for them than leaving a trail of corpses for investigators to follow.

Is the canine intruder actually a puppy? Is there an even more terrifying adult form? Or is this creature trapped in an immature form forever? Is the puppy even the culprit, or is some vicious monster using it as a ventriloquist's dummy with the right combination of spells?

[A G Thing]

While I like the idea it sounds pretty cool already though a very careful encounter with a bit of set up to draw a party into it without making one of them the target of the creature.
Also what purpose other than driving the victim insane does it do this?
To feed off the process?
Because it is like a fiend that wants evil or madness?

Lastly if it is a threat beyond mentally is it able to do so itself or by manipulating others?

Honestly it also made me think a little bit about SCP-637...
https://scp-wiki.wikidot.com/scp-637

[Mephisto of the FoS]

consider this cruel twist in the Demiplane of Dread: the adorable, fluffy puppy that someone has taken into their home actually starts to dominate their life. Not just by the usual (barking, whining, looking cute, making a mess), although it does that as well.

This reminds me of the Ghost Cat aka "unfamilliar" in Children of theNight: Ghosts, the creature can actually take the form of any animal though usually it takes the form of ablack cat, raven, rat or monkey and behaves in an identical fashion to that of the type whose form it has taken. As time goes on, it spends more time staring at its "master" as if that person was prey. The realisation that the unfamilliar is the true master and its apparent "master" no more than a hapless victim, typically drives the target of its attention into acts of desperation. These acts inevitably bring destruction and suffering not just to the victim but to all those who are nearest and dearest to the chosen target.

The following adventure Expiation is one I wanted to use and it is one of the few one-on-one adventures of he setting (if not the only one). The Ghost Cat is one of my favourite (and useful) monsters from the CotN series.

It is also a good NPC to use to any character who has failed a Powers Check, especially if that Player wants to overdo the Powers Check rule system, this way a troublesome Player may come to his/her/their senses in a creative way or end up an NPC and see that making many evil acts to overuse the Powers Check Rule is not so fun in the end. I am saying this because in the past I had Player who acted like that, in the end I turned him into a ghoul (though without the help of the Ghost Cat) and used him as an NPC that schemed and hunted the other players (including the Players new character).

[Manofevil]

I remember reading a book when I was younger about ghost animals including the Demon Cat that haunts the American Capitol Building. One that stuck in my mind was a ghost sheep. Once there was a shepherd who was cruel to his sheep until one day, one of them got angry and head butted him off a cliff. He was part of a large family who just killed the sheep, buried the shepherd, and moved on. Two generations later, a great nephew of the shepherd was seemingly attacked on the same cliffside by another sheep that glowed. He managed to escape twice before the rest of his family told him the story of his ancestor. They solved the problem by stuffing some his clothes with straw and letting the ghost sheep butt the dummy off the cliff, thereby pacifying it.
Ghost animals are doable. I made Buck from "The Call of The Wild" into a ghost animal for Quoth The Raven. I don't know about making them Darklords, but they can at least be great flavor creations.

[Wolfglide of the Fraternity]

This also makes me think of the angoyang from Pathfinder, a false feline manipulator.